1. Field of the Invention
This invention relates to power supplies and DC to DC converter systems utilizing double-ended driven inverter circuits. It is specifically concerned with the prevention of continued magnetic saturation in the core of the inverter power transformer and limitation of its deleterious effects.
Switching devices in double-ended inverter circuits may alternately apply unequal volt seconds to the inverter transformer if they are improperly matched. Such improper matching or mismatches may be due to wide tolerances in the characteristics of the oppositely phased switching devices or may occur due to changes in these characteristics due to the age of the switching device. In other instances these characteristics may change with variation in the operating temperature of the switching device. In any case, if the operating characteristics of the switching devices are not matched, the signal magnitudes conducted by the devices in response to identical driving signals may be unequal.
Such imbalances may occur even if the characteristics of the switching devices are evenly matched. These imbalances may occur due to imbalances in the driving circuitry which biases the switching devices into their conductive condition.
As a consequence of these imbalances in the switching devices or its driving signals, the alternately directed volt seconds supplied to the primary winding of the power transformer are not balanced. The volt seconds applied to the primary winding in one direction are greater in magnitude than the oppositely phased volt seconds applied in the other direction. The magnetic flux produced by these unequally matched, oppositely phased volt seconds during alternate half cycles does not cancel but instead permits the creation of a unidirectional DC flux in the magnetic core of the power transformer. This unidirectional flux increases in magnitude during successive cycles of operation and saturation eventually occurs in the magnetic core.
A consequence of saturation in the transformer core is to permit a large peak transient current to flow through one of the switching devices due to the reduced inductive impedance presented by the transformer. It is not uncommon for this peak transient current to have a magnitude as much as five times the magnitude of the normal load current. This high transient current may cause overheating and degradation in the switching device. In the case where the switching device is a switching transistor, a significant increase in collector load current seriously reduces the operational life of the transistor and may necessitate premature replacement of the switching transistor.
2. Prior Art
Solutions to this saturation problem have been previously devised for application to pulse width modulated converter circuits. In these circuits the oppositely phased currents flowing through the oppositely phased switching devices are monitored and compared. If a current imbalance in the switching devices occurs, the duty cycle of the switching devices is modified to restore the current balance.
Examples of this particular solution are disclosed in H. R. Weischedel et al. U.S. Pat. No. 3,870,943, issued Mar. 11, 1975, and in R. A. Reed U.S. Pat. No. 3,859,583, issued Jan. 7, 1975. Both of these patents are assigned to the same assignee to which the present application is assigned.
This solution has limitations. It is limited in application to pulse width modulated converter circuits. Furthermore, it measures only the current magnitude and balance in the alternately phased switching devices as opposed to responding to the condition of saturation in the transformer. The volt seconds balance of an applied signal is a more accurate measure of saturation than the current magnitude balance. Hence, in these circuits the current magnitude may be balanced and saturation may still occur in the transformer.
Another solution of the transformer saturation problem is to protect the switching devices independently of the condition of saturation in the transformer. In this approach currents flowing in each switching device are monitored and compared with a predetermined current limit. When this current limit is exceeded, current conduction in the switching device is terminated. A solution of this type is disclosed in D. M. Mitchell et al. U.S. Pat. No. 3,927,363, issued Dec. 16, 1975.
The Mitchell patent discloses a self-oscillating inverter circuit having a normally saturating transformer. It includes switching transistors which are cut off in response to a protection circuit which responds to a particular threshold of the collector-emitter current in the switching transistor. The purpose of this protection circuit is to limit current flow through the switching transistors and not to prevent transformer saturation.
It is apparent from the foregoing discussion that an adequate protection circuit to prevent saturation of the inverter transformer must operate to respond to the actual condition of saturation in the core of a transformer and not just to a current balance or a predetermined current limit in the switching devices.